Elevated cholesterol levels are an important risk factor for cardiovascular disease, which is a leading cause of death in the industrialized world. Controlling the risk factors for cardiovascular disease is a key preventive and therapeutic target in reducing this high mortality rate. Well-established risk factors for cardiovascular disease include elevated plasma levels of cholesterol (hypercholesterolemia), triglycerides, homocysteine, and certain lipoproteins, such as low-density lipoprotein (LDL) cholesterol.
In particular, an elevated level of LDL cholesterol is an independent risk factor for premature coronary heart disease (CHD), with a value of 160 mg/dl or greater designated as high-risk by the National Cholesterol Education Program Adult Treatment Panels I, II and III. Current goals of therapy for all patients with elevated LDL cholesterol include reducing levels to: (i) less than 160 mg/dl in those with at least one or more CHD risk factors; (ii) less than 130 mg/dl in those with 2 or more CHD risk factors; and (iii) less than 100 mg/dl in patients with established CHD or CHD risk equivalents, one of which is diabetes.
Elevated intermediate-density lipoprotein (IDL) levels are another independent risk factor for cardiovascular disease. (see, e.g., Chapman et al. Clin Cardiol. (2003) 26(1 Suppl 1):I7-10; Shoji et al. J Am Soc Nephrol (1998) 9:1277-84; Nordestgaard et al. Eur J Epidemiol. 1992 May; 8 Suppl 1:92-8. Levels of apoB (the main protein component of LDL) and apoAI (the main protein component of HDL) represent additional risk factors for cardiovascular disease. See, e.g., Walldius et al. J Intern Med. (2006) 259(5):493-519; Charlton-Menys et al. J Intern Med. (2006) 259(5):462-72; Walldius et al. Clin Chem Lab Med. (2004) 42(12):1355-63. Stated simply, LDL and IDL are known to have atherogenic properties in patients. In contrast, HDL and ApoA1 are known to have anti-atherogenic properties.
The discovery of drugs that inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) was a major advance in the treatment of patients with elevated plasma concentrations of LDL cholesterol. The efficacy of HMGCR inhibitors (statins) in LDL-lowering and CHD risk reduction has clearly been demonstrated in a number of primary and secondary intervention trials. (For a review, see, e.g., Brousseau, IDrugs. (2003) 6(5):458-63).
HMGCR catalyzes the conversion of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) to mevalonate in the rate limiting step of biosynthesis of isoprenoids, a class of compounds involved in sterol synthesis (e.g., cholesterol synthesis) as well as other cellular functions. Because of its role in cholesterol synthesis, HMGCR inhibitors have been of particular interest in controlling cholesterol levels. Statins, one of the most widely prescribed classes of drugs in the US, directly inhibit HMGCR activity, and the resulting reductions of blood levels of LDL cholesterol have been shown to substantially reduce risk for coronary heart disease.
At least four isoforms of HMGCR have been described (see, e.g., WO 03/102209, and Keller et al., 1986 J Cell Biol 103(3): 875-86). The classical isoform of the HMGCR enzyme is a transmembrane-protein anchored to the endoplasmic reticulum, sometimes referred to as the “ER HMGCR” or “full-length HMGCR”, and is encoded by the complete transcript of the HMGCR gene in humans (see, e.g., GenBank Accession No. NM—000859). The HMGCR isoform found in peroxisomes has been reported to be resistant to statin inhibition (Aboushadi et al., 2000 Biochemistry 39(1): 237-47). The peroxisomal HMGCR, which the inventors suggest here is a splice variant of the ER HMGCR isoform in humans, lacks exon 13 (Johnson et al., 2003 Science 302(5653): 2141-4; GenBank Accession no. BC033692). Peroxisomal HMGCR is involved in biosynthesis of cholesterol, plasmalogen and bile acids and oxidation of fatty acids. Patients having peroxisome deficiencies (PD) have a reduced rate of cholesterol synthesis (approximately 16-20% of normal) and abnormally low plasma cholesterol concentrations. PD patients also exhibit reduced uptake of LDL-cholesterol due to abnormal LDL synthesis. Furthermore, there is reduced LDLR recognition of LDL produced by PD patients.
The wide range of inter-individual efficacy of HMGCR inhibitor drugs (statins) is a major issue in treatment of patients in need of cholesterol-lowering therapy. Studies have reported that the decrease in LDL-cholesterol in similar individuals taking the same type and dose of statin, adjusted for compliance, range from less than 5% to greater than 60%. (1998. Influence of pravastatin and plasma lipids on clinical events in the West of Scotland Coronary Preventive Study (WOSCOPS). Circulation 97:1440-1445; Sacks et al. Circulation 97:1446-1452) Reduced statin responsiveness likely contributes to the observation from clinical trials that 70-80% of clinical events are not prevented by statin treatment. (1998. Influence of pravastatin and plasma lipids on clinical events in the West of Scotland Coronary Preventive Study (WOSCOPS). Circulation 97:1440-1445 MRC/BHF Heart Protection Study of cholesterol lowering with simvastatin in 20 536 high-risk individuals: a randomised placebo controlled. Trial. Heart Protection Study Collaborative Group. Lancet 2002; 360: 7-22).
Several investigators have attempted to explain the variability in statin response by evaluation of genetic markers such as single nucleotide polymorphisms (SNPs) in the HMGCR gene (Chasman et al., (2004) JAMA 291(23): 2821-2827; US 2003/0215819; WO 00/79003). However, SNPs that are known to be associated with statin pharmacogenetics are relatively rare, and therefore only useful for predicting efficacy in a small percentage of the population.
Thus, there is a need for a test to evaluate an individual's sensitivity to HMGCR inhibitors, such as statins. A test to identify those patients most likely to benefit from such a treatment would represent a substantial improvement in the ability to reduce cardiovascular risk in the population, while avoiding unnecessary and even counterproductive administration of drugs that do not provide the desired therapeutic result. There is also a need for methods for identifying drugs that can modulate cholesterol levels (e.g., decrease LDL cholesterol levels, decrease IDL cholesterol levels, and/or raise HDL-cholesterol levels), either when administered alone or in conjunction with a statin, with improved efficacy, particularly in patients who fail to reach the desired therapeutic targets with statin treatment. The present invention addresses each of these needs.